A carbon nano tube (i.e., CNT) is synthesized by an arc discharge method, a laser vaporization method, or a chemical vapor deposition method. In view of productivity, the CVD method becomes mainstream.
The CVD method is classified to a fluidized bed CVD method and a fixed bed CVD method. The fluidized bed CVD method is such that raw material gas including carbon for synthesizing the CNT and a CNT growth catalyzer for functioning as reacting origin are introduced into a heated reactor at the same time. The fixed bed CVD method is such that a substrate is arranged in a heated reactor, in which a catalyzer is arranged, and then, raw material gas is introduced into the heat reactor.
A fixed bed CVD device includes a raw material gas supplier, a carrier gas supplier, an electric furnace for generating heat, a substrate for holding the catalyzer, and a reactor tube. If necessary, the fixed bed CVD device further includes a vacuum pump for pumping air from the reactor tube, which is arranged on a downstream side of the reactor tube. This type of CVD device is disclosed in, for example, JP-2006-62924. In this device, the CNT can be synthesized on the substrate. In an initial synthesizing step, the length of the CNT becomes large in proportion to a process time. However, the length of the CNT is saturated after the process time of a few minutes to a few tens of minutes has passed. Thus, the maximum length of the CNT is a few hundreds microns to a few millimeters. Accordingly, the CNT cannot be synthesized to have the length longer than the maximum length.
To form the CNT having large length, a two-step synthesis method is disclosed in, for example, JP-2005-29414 corresponding to USP Application Publication No. 2005/0013762.
However, in the two-step synthesis method, caulking phenomenon may occur under some conditions of raw material gas. The caulking phenomenon is such that a carbon film coats the surface of the catalzser. Here, the carbon film is not made of the CNT. When the carbon film covers the whole surface of the catalyzer, carbon supply to the catalyzer stops, so that the catalyzer does not function, i.e., the catalyzer is deactivated. Thus, the growth of the CNT stops.